1. Field of the Invention
This invention relates to a roller mill operating system for supplying fuel to a burner to control the emission of nitrous oxide.
2. Description of the Prior Art
In boiler applications for burning of pulverized coal a standard has been developed to limit, if not eliminate, the emission of nitrous oxide into the atmosphere. The manufacturers of boilers must provide continuous monitoring systems that summate nitrous oxide emissions over a given period of time with the result that penalties can be assessed, or a plant can be shut down, if standards are not met.
Similarly the manufacturers of coal burning devices who supply such devices for use in boiler equipment are specifying that the coal must be ground to a particle size of 50 percent, minus 1000 microns all the way down to 70 percent, minus 74 microns, and the conveying air must be limited to an amount of substantially two pounds of air per pound of coal, or lower as that ratio of air to coal and coal size or fineness will enable the burner to meet the nitrous oxide standard. The manufacturers have stated that if coal is ground too fine, such as 80 percent, minus 74 microns, the combustion is so rapid that nitrous oxide is created, and if the coal is too coarse, the boiler has a problem of formation of slag.
Conventional coal pulverizers have a fixed area in the grinding chamber and a certain fixed air velocity is required to produce 70 percent, minus 74 micron coal particle size, and that air velocity is about 1100 F/M through the grinding zone. The quantity of air required for such a system is the product of velocity times open area which is a standard CFM that is used to size, dry, and convey the 70 percent, minus 74 micron particles out of the grinding system and into the boiler. From the above equation it is obvious that in a direct firing pulverizer system, the air leaving the system must go to the boiler, and that limits the ability to particularly match the grinding fineness to the air to obtain a 2:1 or optimum ratio of air to coal. This is particularly true when a boiler turndown is considered, as it is virtually impossible to maintain the air velocity through the grinding zone sufficiently high to produce a product of 70 percent, minus 74 microns and not exceed a 2:1 ratio of air to coal.